Update to support netstandard/Azure Functions V2

[Morcatko]

Hey, great work. I love this wrapper and I would like to use it in my app.
I tried to use your library on Azure Functions V1 (.NET framework) and it works great.
I only had to remove PdfiumViewer.Native nuget and copy DLLs manually as the Functions runtime uses slightly different folder structure then the default Visual Studio bin output.

My questions is if you would be willing to update your library to support netstandard (and also Azure Functions V2). It would mean some small breaking changes but I believe it would highly increase the list of supported scenarios.

Here is what I beleive needs to be done

• Remove dependency on PdfiumViewer.Native. This is Azure Functions specific and can be workarounded by some copy/delete in a build script so not a must have. (VS build copies DLLs to bin folder, but AzFunctions require it to have in "bin/netstandard2.0/bin")
• Remove dependency on System.Drawing. This is currently a blocker that limits the library to Windows. System.Drawing is Windows/.NET framework specific library that does not work in netstandard (There is a way how to run it in .NET Core but it is not 100%). It would be nice if PdfiumLight only fills some byte buffer with pixel data and external code passes this buffer to image library of choice. Something that https://github.com/ArgusMagnus/PDFiumSharp does - have independent PdfiumLight library + PdfiumLigh.System.Drawing, PdfiumLigh.ImageSharp, ....

I can make some PRs if you agree to this change.

[marcpabst]

Hey, thanks for your kind words.
Please feel free to make any PRs you like.

I'm a little short on time at the moment, so I'm afraid I won't be of much help here.